Also, could it be that the IMEI is not registered on the account.. If you just swap, sometimes it does not work.. I know because i used to work with AT&T and had tons of customers with these problems. Access point should solve it, but if it doesn't call CS

This is where marketing has made things confusing.
In the UK we have 2G (voice and text) 3G, including 3G+ and H, H+ (internet etc) and 4G (faster Internet etc)
In the US (AT&T at least) the have called H and H+ 4G and what we call 4G they have called 4G LTE, so in access point it only says 3G but if the front of the phone says 4G you will be getting the faster speed just not LTE which the phone doesn't support.
Clear as mud